Has nothing to do with spam and everything to know who you are. Spammers don't use "burner phones".

Spam was never done with "burner phones" in the first place, it's mostly done via VoIP through shady telecoms companies that can't be bothered to validate their customers. Due to the age of the phone system it's incredibly easy to spoof phone numbers because it's essentially a trust system. Phone exchange A talks to exchange B and says phone number 123 is calling number 456. How does exchange B know that it's actually 123 calling? They don't at all, they just trust that exchange A is telling the truth. It's really hard to get into the system, but once you're there you essentially have unlimited power with virtually no safeguards in place.

Basically from a security perspective the phone system looks a lot like the 1980s internet, there is technically some security in place, but significantly less than there actually should be.

This isn’t to fight spam, it’s to fight the populace